Polishing method for inner surface of tubular brittle material and tubular brittle material obtained by polishing method

ABSTRACT

An object of the present invention is to provide a polishing method for producing, in a relatively short period of time, a tubular brittle material having an inner surface of high surface precision with a maximum roughness R max  of 0.1 μm or smaller and a center line average roughness R a  of 0.01 μm or smaller, and to provide a tubular brittle material with high precision using said polishing method. The polishing method according to the invention is characterized by that the inner surface of the tubular material being pre-cut into a tubular shape using a honing machine is further polished with a sheet material having diamond abrasives attached thereon. Also claimed is a tubular brittle material obtained by said polishing method.

INDUSTRIAL FIELD OF APPLICATION

The present invention relates to a polishing method for an inner surfaceof a tubular brittle material and the tubular brittle material to beobtained by the polishing method and, particularly, it relates to amethod for polishing an inner surface of a quartz glass tube for use inproduction of an optical fiber with a good surface accuracy and thequartz glass tube having a high inner surface accuracy.

PRIOR ART

A tubular brittle material, particularly, a high-purity quartz glasstube has been used as a reaction tube of an inside vapor depositionmethod (MCVD method) for producing an optical fiber, or has been used asa jacket tube of a pre-preform produced by any one of the followingmethods: A Modified Chemical Vapor Deposition Method (MCVD method), anAxial Vapor Deposition Method (VAD method), and an Outside VaporDeposition Method (OVD method). When irregularities or cracks arepresent on an inner surface of the high-purity quartz glass tube used asa jacket tube of a pre-preform, air bubbles are generated at the timethe pre-preform and the quartz glass tube are melt-integrated with eachother whereupon the thus-generated air bubbles cause problems of a wirebreakage at the time of wire drawing, a contact defect of the opticalfiber and the like. For these reasons, it is necessary to allow asurface accuracy of an inner surface of the quartz glass tube to behigh. In JP-A No. 2000-119034 it is suggested to subject the innerperipheral surface of a high-purity quartz glass ingot to a mechanicalgrinding treatment by a diamond whetstone, and then the inner surfacethereof is subjected to a mechanical polishing treatment by a whetstonemade of cerium oxide.

PROBLEMS THAT THE INVENTION IS TO SOLVE

However, in a method described in the above-cited patent document, itoccurs that a large fragment comes off an edge portion of a whetstonemade of cerium oxide at the time of polishing and, the thus-generatedfragment was seized in a space between the whetstone and a polishedsurface to cause a scratch on an inner surface of an article to bepolished. Aiming at preventing generation of such scratch, a method inwhich abrasive grains made of cerium oxide are first dispersed in agrinding liquid and, then, a surface to be ground is subjected togrinding by a brush or the like has been developed. However, in thiscase the period of polishing time was extremely long so that the methodwas inferior in productivity. Further, another method has been developedin which a sheet of sheet material to which the abrasive grains made ofcerium oxide were adhered (hereinafter referred to also as “cerium-oxidepaper”) is used. However, the cerium-oxide paper was low in an initialpolishing ability, namely, the ability of grinding irregular portions bygrinding at a preliminary step was low whereupon, not only it isnecessary to allow roughness of the inner surface to be smaller at suchpreliminary polishing step, but also it is necessary to frequentlychange the cerium-oxide paper to a new one due to a heavy attrition;therefore, there was a problem of a high cost.

Therefore, it is an object of the present invention to provide apolishing method capable of performing polishing with a high surfaceaccuracy without generating a scratch on an inner surface of a tubularbrittle material.

Further, it is another object of the invention to provide a polishingmethod capable of reducing a polishing time.

Furthermore, it is still another object of the invention to provide atubular brittle material having a high inner surface accuracy.

MEANS FOR SOLVING THE PROBLEMS

Under these circumstances, the present inventors have conducted anintensive study and, as a result, have found that, when the innersurface of the tubular brittle material which has been subjected to apreliminary grinding treatment using a honing machine is polished byusing a sheet material on which diamond grains are attached (hereinafteralso referred to as “diamond paper” or as “diamond sheet material”). Theinner surface roughness obtained by using a “diamond paper” for thefinal polishing is the same as or even better than that which isobtained by performing polishing by using cerium oxide and, further,polishing time can substantially be reduced, thereby achieving thepresent invention.

Therefore, the present invention which attains the above-describedobjects relates to a method for polishing an inner surface of a tubularbrittle material with a good surface accuracy which is characterized inthat the inner surface of a brittle material which has beenpreliminarily ground into a tubular shape by a honing machine which isequipped with a polishing head and after that it is polished by adiamond sheet material. Furthermore the invention relates to a tubularbrittle material having a high inner surface accuracy.

The term “brittle material” herein used is intended to include amaterial which has a small breaking strain when the material is allowedto be broken by an external force whereupon specific examples of suchbrittle materials include glass, and ceramics. In order to form atubular body from the brittle material, a diamond whetstone is set on ahoning machine and a grinding operation is started. As the grindingoperation proceeds, diamond grains come off the whetstone and grainscome off the article to be ground when it is ground adhere to a surfaceof the whetstone to cause clogging. When the clogging is generated, notonly grinding ability is decreased, but also resistance between thewhetstone and the article to be ground is changed whereupon a vibrationis generated. In an extreme case, a large stress comes to be added tosometimes break the whetstone. This feature is conspicuous whenpolish-finishing is performed by using the whetstone having a smallergrain size. On the other hand, in a case in which sheet material adheredwith diamond abrasive grains is used in place of the above-describeddiamond whetstone, it is found that, even when clogging is generated,since the sheet material itself has elasticity, no vibration isgenerated and, also, no breakage of sheet material is generated, therebycontinuously performing a polishing treatment. It is also found that,when polishing is continuously performed in such a clogged state asdescribed above, polishing proceeds more favorably than expected with agiven gauge of the abrasive grains. It is considered that, whenpolishing of the brittle material is continuously performed while thediamond sheet material is clogged, an entire surface of the sheetmaterial is covered by grains come off the article to be polished whenit is polished and, since the grains are made of same material as thatof the article to be polished, a so-called ground-in state is generatedtherebetween and, then, polishing grains themselves are ground to begradually made smaller in size, thereby being capable of attaining afavorable polishing. When the diamond sheet material is used in such amanner as described above, since the tubular brittle material having ahigh surface accuracy can be obtained, a period of polishing time cansubstantially be reduced without necessity of changing diamond abrasivegrains into those having a smaller gauge.

The base material of the “diamond sheet material” may be paper, textileor a plastic foil. The surface on which the diamonds are attached may beeven, corrugated or it may show knobs.

Although a honing machine equipped with a polishing head is ordinarilyused for polishing the tubular brittle material, it is difficult toexpand the polishing head such that the polishing head is pressedagainst the entire inner surface of the brittle material whereupon thepolishing head is partially expanded. Then, a whetstone is attached tothe thus-expanded portion. When the tubular brittle material is polishedby the honing machine attached with this polishing head, an edge portionof the whetstone is caught by a surface to be ground to sometimesgenerate a large fragment. The thus-generated fragment is seized betweenthe edge portion of the whetstone and the surface to be ground to give adeep scratch on the surface to be ground. Therefore, it is preferredthat the entire polishing head is covered with the diamond sheetmaterial, even when an edge is present on the expanded portion, sincethe diamond sheet material is continuously provided, an intrusion of thefragment does not occur whereupon no scratch is generated on the innersurface of the brittle material.

A grain diameter of the diamond of the above-described diamond sheetmaterial to be used is preferably in the range of from #500 to #10000(the sign # represents the mesh size of the grains attached to the sheetmaterial).

In a preferred embodiment of the method according the invention, thesheet material having diamond abrasives attached thereon is fixed to thepolishing head by using a hook and loop fastener. The hook and loopfastener facilitates the replacement of used diamond coated sheets.

It has been found advantageous if the diamond abrasives are attached ona surface of the sheet material which is knobbed or corrugated. Such asurface is allows a flow of fluids, which are used as an auxiliarypolishing agent for transporting and removal of the abraded glassparticles.

MODE FOR CARRYING OUT THE INVENTION

The present invention will further be explained specifically withreference to embodiments to be described below, but is by no meanslimited to these embodiments.

A maximum roughness R_(max) and a center line average roughness R_(a)were measured on every 10 mm long in accordance with definitions set byJapanese Industrial Standards (JIS) B0601 whereby a contact-typesimplified surface roughness meter was used for performing themeasurement (Model: Surfcom 300B; manufactured by Tokyo Seimitsu Co.,Ltd.).

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematically drawn diagram showing the polishing methodaccording to the present invention,

FIG. 2 is a schematically drawn diagram showing, a polishing head havingan extended part in 4 directions,

FIG. 3 is a schematically drawn diagram showing a polishing head havinga diamond sheet material wound around the entire extended part.

EMBODIMENTS Example 1

Silicon tetrachloride was vaporized and, then, the thus-vaporizedsilicon tetrachloride was subjected to flame hydrolysis in an acidhydrogen flame and, thereafter, silica glass fine particles were allowedto be deposited on a surrounding area of a rotating substrate. Alarge-size porous soot body was obtained according to the OVD method.The thus-prepared porous soot body was put in an electric furnace and,while taking conditions such as a refractive index and the like of acore glass rod into consideration, dehydrated by heating at 1100° C. bya mixed gas of He and Cl₂ and, subsequently, allowed to be transformedinto a transparent glass by heating at 1600° C. in an atmosphere of Heto produce a cylindrical quartz glass ingot. Both ends of thethus-produced cylindrical quartz glass ingot were cut and, then, aninside thereof was ground by a vertical honing machine attached with acylindrical polishing head as shown in FIG. 2 having expanded portions 3(machine hones) which are distributed uniformly around the mantelsurface and which extend along its entire length. The expanded portions3 have a size of 25 cm long and 5 mm wide and they are set with a #800resin-bonded diamond whetstone to prepare a quartz glass tube having aninside diameter of 50 mm and a length of 2 m. The entire polishing headincluding the expanded portions 3 was wrapped around by a diamond paper4 and fixed thereon by a hook and loop fastener as it is shown in FIG.3. The base material of the diamond paper 4 is a textile material havinga corrugated surface on which diamond grains are attached. Withreference to diamond grain diameters of the diamond paper 4 which wasused for wrapping, #1200 was used at first, #2000 secondly and #3000lastly by changing papers three times from one another.

The polishing head was inserted in the quartz glass tube andreciprocated 80 times along an entire length of the quarts glass tubewhile the polishing head was rotated at 100 rpm and moved at a speed of3 m/min, as it is shown in FIG. 1. The maximum roughness R_(max) and thecenter line average roughness R_(a) of the inner surface of thethus-obtained quartz glass tube were 0.08 μm and 0.007 μm, respectively.

Time consumed for the polishing treatment was slightly less than 6 hoursincluding time for change-over of diamond papers. When the diamond paperhaving grain size of #6000 was further reciprocated 80 times on theinner surface of the quartz glass tube, the maximum roughness R_(max)and the center line average roughness R_(a) of the inner surface of thequartz glass tube became 0.06 μm and 0.005 μm, respectively.

Example 2

Polish-finishing was performed in a same manner as in Example 1 exceptthat the diamond paper 4 was attached only on the four expanded portions3 (machine hones) which are distributed uniformly around thecircumference of the polishing head 4. The maximum roughness R_(max) andthe center line average roughness R_(a) of the inner surface of theobtained quartz glass tube were approximately same as those in Example1; however, one helical scratch was observed which was conceivablycaused by allowing a fragment of the whetstone or the quartz glass to besiezed was generated on the inner surface. Therefore, the result isacceptable only for low quality requirements and this is not the bestmode for performing the method according to the invention.

Comparative Example 1

In order to perform polishing of the inner surface of the quartz glasstube having an inner diameter of 50 mm and a length of 2 m which hasbeen obtained by grinding the quartz glass ingot having a same size asthat in Example 1, a head of the honing machine attached with a nylonbrush was set inside the quartz glass tube and, then, while a polishingliquid which has been obtained by mixing abrasive grains made of ceriumoxide in pure water was flowed inside the tube from an upper portionthereof, the above-described brush was continuously reciprocated 240times (80 reciprocations by three times) along an entire length of thequartz glass tube while the brush was rotated at 500 rpm and moved at aspeed of 3 m/min. The maximum roughness R_(max) and the center lineaverage roughness R_(a) of the inner surface of the obtained quartzglass tube were 0.5 μm and 0.2 μm, respectively. In order to allow theinner surface accuracy of the thus-obtained quartz glass tube to be sameas that of the quartz glass tube in Example 1, it was necessary toconsume slightly less than 22 hours in conducting 960 reciprocations.

Comparative Example 2

Same procedure was repeated as in Example 1 except that cerium-oxidepaper was used in place of the diamond paper and one cycle of polishingin which the polishing head was reciprocated 80 times along an entirelength of the quartz glass tube while the polishing head was rotated at100 rpm and moved at a speed of 3 m/min was repeated three times whilepapers were changed each time. The maximum roughness R_(max) and thecenter line average roughness R_(a) of the inner surface of the obtainedquartz glass tube were 0.35 μm and 0.15 μm, respectively. In order toobtain same inner surface as in Example 1 by this polishing method, itwas necessary to repeat a cycle of 80 reciprocations 7 times whilepapers were changed each cycle. Time consumed for the polishingtreatment was about 13 hours including time consumed for change-over ofpapers.

ADVANTAGE OF THE INVENTION

According to the polishing method of the present invention, a quartzglass tube having a high surface accuracy in which a maximum roughnessR_(max) and a center line average roughness R_(a) of the inner surfacethereof are 0.1 μm or less and 0.01 μm or less respectively can beproduced in a relatively short period of time and, then, by using thethus-produced quartz glass tube as a jacket tube of a pre-preform or areaction tube of an MCVD method, a high-quality optical fiber can easilybe produced.

Explanation of Symbols

1: Quartz glass tube

2: Polishing head

3: Extended part

4: Diamond paper

5: Shank bar

6: Pushing rod

1. A method for polishing an inner surface of a tubular brittle materialat high surface precision, said method comprising: subjecting the innersurface to a preliminary grinding treatment using a honing machine whichis equipped with a polishing head, and after that polishing the innersurface with a sheet material having diamond abrasives attached thereon.2. The polishing method as claimed in claim 1, wherein the sheetmaterial having diamond abrasives attached thereon is wound around thepolishing head of the honing machine.
 3. The polishing method as claimedin claim 2, wherein the sheet material having diamond abrasives attachedthereon is fixed to the polishing head by a hook and loop fastener. 4.The polishing method as claimed in claim 1, wherein the diamondabrasives have a grain diameter in the range of from #500 to #10000. 5.The polishing method as claimed in claim 1, wherein the diamondabrasives are attached on a surface of the sheet material which isknobbed or corrugated.
 6. A tubular brittle material having high innersurface precision obtained by the polishing method as claimed in claim1, having an inner surface precision with a maximum roughness R_(max) of0.1 μm or smaller and a center line average roughness R_(a) of 0.01 μmor smaller.
 7. A tubular brittle material as claimed in claim 6, whereinthe tubular brittle material is a quartz glass tube for use in producingoptical fibers.
 8. The polishing method as claimed in claim 2, whereinthe diamond abrasives have a grain diameter in the range of from #500 to#10000.
 9. The polishing method as claimed in claim 3, wherein thediamond abrasives have a grain diameter in the range of from #500 to#10000.
 10. The polishing method as claimed in claim 2, wherein thediamond abrasives are attached on a surface of the sheet material whichis knobbed or corrugated.
 11. The polishing method as claimed in claim3, wherein the diamond abrasives are attached on a surface of the sheetmaterial which is knobbed or corrugated.
 12. The polishing method asclaimed in claim 4, wherein the diamond abrasives are attached on asurface of the sheet material which is knobbed or corrugated.
 13. Atubular brittle material having high inner surface precision obtained bythe polishing method as claimed in claim 2, having an inner surfaceprecision with a maximum roughness R_(max) of 0.1 μm or smaller and acenter line average roughness R_(a) of 0.01 μm or smaller.
 14. A tubularbrittle material as claimed in claim 13, wherein the tubular brittlematerial is a quartz glass tube for use in producing optical fibers.